Most large-scale applications of high-Tc superconductors (HTS) demand high critical current densities (Jc) and strong superconducting coupling across grain boundaries. It is known that grain boundaries in HTS (e.g., YBa2Cu3O7-x, BiCaSrCuO, etc.) have an adverse effect on the transport properties. This effect is normally worsened when the grain misorientation increases, except for some special grain boundaries. It has been shown that Jc across a grain boundary decreases exponentially as the misorientation angle (θ) of the boundary increases. Several models have been proposed to account for this characteristic grain boundary behavior. In general, the presence of grain boundary dislocations has been used to explain the reduction of Jc for small-angle grain boundaries (θ<15°). High angle grain boundaries contain a disordered layer whose effective width was found to increase linearly with increasing misorientation. It has been suggested that the exponential decrease of Jc is associated with this increase.
In addition to the misorientation angle, the inclination of the boundary plane, which defines the orientation of the main crystal axes with respect to the grain boundary, has an overriding influence on the critical current density. The Jc value could vary by several orders of magnitude for a change of the boundary inclination of a given misorientation angle. Different from conventional superconductors, the high temperature superconducting cuprates have d-wave symmetry, which implies that the variation in inclination of the grain boundary plane alters the transport current across the interface. Using two extreme cases, it has been shown that the critical current density across the boundary can change from the Sigrist-Rice clean limit (facet free) to its dirty limit (maximum random facets). It has been urged that the facets along grain boundaries tend to alter the path of current transport since the inclination essentially determines the directionality of couplings across the boundary. The critical current density rapidly decreases with misorientation and has been explained by the combined effect of the d-wave pairing symmetry and the observed boundary meandering. However, the Jc values obtained from these facet-related models still fail to match the experimental data of the rapid decrease of Jc with misorientation angle. To explain these discrepancies, it has been proposed that the grain boundary plane can only take on discrete hkl inclinations and not arbitrary orientations during faceting. It has been suggested that the discretization of the boundary inclination is a result of a discrete crystal lattice and the nonperiodic nature of the faceting represents a more realistic boundary structure. Theoretically, the boundary inclination planes take the crystallographic planes with low hkl values from the discrete lattice point of view, depending on the misorientation of neighboring grains, chemical segregation and other effects.
While the electrical properties of the grain boundaries have been extensively studied, the microstructure of the HTS grain boundaries has not been well characterized. Many microstructural studies of the epitaxially grown grain boundaries have focused on the meandering nature of the film grain boundaries. The grain boundary meandering means a continuous change of the grain boundary plane, which results from the twists and turns of boundary at the microscopic scale. It is the faceting that defines the atomistic crystallographic plane of the grain boundary and thereby the change in the path of current transport. Although the observation of the nanoscale facets at grain boundaries of YBCO thin films grown on STO substrates has also been reported, the faceting mechanisms was not well understood and it was also not clear if the grain boundary faceting is a general and pervasive phenomenon.
Since many electronic devices that use superconductors require reproducible junction currents, the ability to produce uniform junctions is essential. In high temperature super-conducting oxides, such as YBa2Cu3O7-x and BiCaSrCuO, the coherence length is very small comparable to the lattice parameters. Therefore, the atomic structure of the junctions is a controlling factor of the junction properties. The origin of the deviation of the electronic properties along a high angle grain boundary in high temperature superconducting films derives from the microscopic meandering and the nanoscopic faceting of the boundary. This meandering does not derive from the template bicrystal substrates, but from island growth mode of the films on the substrate surface resulting from the chosen deposition methods. Since off stoichiometric deposition of the oxide films is generally detrimental to superconducting properties, a deposition method must be used that allows congruent transfer of stoichiometric materials. In order to prepare a non-meandering boundary, not only does the template boundary need to be straight, but the thin film growth mode needs to be a layer-by-layer type. Therefore, a need exits in the art for a method of preparing thin film oxides deposited on a substrate with a bicrystal grain boundary that is completely straight and planar on a microscopic and atomistic level so the d-wave coupling across the boundary will not vary along the boundary, allowing the preparation of thin film oxides with a reproducible junction current.